1. Field of the Invention
The present invention relates generally to apparatus and methods for producing steam, and more particularly, but not by way of limitation, to apparatus and methods for producing steam from low quality feedwater including oily saline water produced from an underground oil and gas production zone.
2. Description of the Prior Art
One manner of stimulating the production of hydrocarbons from subsurface formations is to inject steam into the subsurface formation. An oil production operation based on such steam injection normally requires a source of high quality feedwater for steam generation and also requires a means for disposing of oily saline produced water which is recovered from the formation along with the hydrocarbons.
In some areas, however, a reliable supply of high quality feedwater is not readily available, and it is, therefore, desirable to recycle oily saline produced water and utilize the same to generate steam to be injected back into the underground formation.
Several prior art systems have provided means for generating steam from low quality feedwater or otherwise relate to some part of the steam production system of the present invention as described below.
U.S. Pat. No. 2,947,689 to Cain discloses a process and system for generating hot processed water from feedwater having scale forming salts therein. As illustrated in the figure of the Cain disclosure, that reference discloses a process wherein low quality feedwater 14 is charged to a heating tower 10 first contacting hot flue gas from a superheater, then hot gases from a burner 16, precipitating out sludge at 17. The heated water is acidified and pumped by pump 21 to heating vessel 20 by outlet 24. Steam from heating vessel 20 exits by outlet 26 and is preheated by an exchanger 31 in superheater 29 prior to pressuring up by compressor 33 and superheating in exchanger 34 in superheater 29. The superheated steam produced thereby flows from line 25 to heater 20 to form more steam, exiting by line 26, by a gas liquid contact with the water in heater 20. This process differs substantially from the present invention in that it produces hot water rather than steam.
British Pat. No. 669,928 discloses a system for making distilled water from low quality water such as sea water. Sea water enters pre-heater 4 by inlet 3 and is heated by way of a gas liquid interface by superheated steam entering by inlets 5. Sludge from scaling minerals is disposed of by outlet 6. The heated brine then passes through a distillation process and a portion of the steam generated during that distillation process is superheated in superheater 21 and recycled to the preheater 4 by the outlets 5.
U.S. Pat. No. 3,410,796 to Hull and an article entitled "The Thermosludge Water Treating and Steam Generation Process" from the December, 1967 of the Journal of Petroleum Technology at pages 1537-1540 thereof, describe a process generally referred to as the "thermosludge" process. Referring to the Hull patent, low quality feedwater enters the system by line 12 and scaling elements are removed as sludge from water feed tank 10 which is heated by recycled steam from line 40. Hot feedwater having much of the mineral content thereof removed passes by line 14 to stripper 24, and is treated with sulfite and amine on the way. Steam is generated by line 26, from stripper 24, which conducts the produced steam to the point of use thereof. Blowdown from the stripper 24 and steam drum 30 passes by outlet 34 through a low pressure separator 38 which drops out more sludge of precipitated minerals by means of line 42, and which passes the steam content of the blowdown to the water feed tank 10 by line 40. The stripper 24 and steam drum 30 are heated by a thermosiphon system wherein water passes by line 32 to a steam chest where a heat exchanger 44 transmits heat to the steam from a molten salt circuit. Although the system illustrated in the Hull reference does produce steam as opposed to merely producing water, its manner of doing so is such that the heat exchanger tubes within the heaters are in contact with hard water and scaling is a problem as is disclosed in column 6, lines 48-56 of the Hull patent.
U.S. Pat. No. 2,756,208 to Axelrad et al. discloses a process for producing hot water utilizing high pressure steam from a conventional boiler to contact water and heat the same.
Another prior art process generally known as the "vapor therm process" is described in ERDA publication No. 10 entitled "Enhanced Recovery of Oil and Gas" at pages 55-57 thereof, and is also described in Report Number 72 of the "First International Conference on the Future of Heavy Crude and Tar Sands", entitled "The Vapor Therm Process for Recovery of Viscous Crude Oil" by F. S. Young, Jr. and R. W. Krajicek. The vapor therm process includes a high pressure air compressor, a high pressure combustion chamber, a water chamber, a water injection and blowdown drum and related pumps and instrumentation. The high pressure combustion gas is contacted with low quality feedwater to generate steam. The mixture of steam and combustion gases is then injected into the wells to perform the flooding operation. Blowdown water including precipitated mineral solids is withdrawn from the contactor.
Another prior art process developed by Esso Resources and described in Application No. 770866 to the Energy Resources Conservation Board, May 1978, includes the generation of superheated steam in a utility type boiler and the subsequently blending of the superheated steam with heated produced brine. Approximately one barrel of high quality water is required for each barrel of saline water to be recycled. The saturated steam generated by the blending of the superheated steam with produced brine is then injected in the subsurface formation.